The space shuttle fleet has a long history of impact damage during lift off, NASA has acknowledged, with Columbia suffering similar incidents at least twice before.
The main line of enquiry for the investigators searching for the cause of Columbia鈥檚 catastrophic disintegration is that a piece of insulating foam dislodged from the external fuel tank during launch damaged the spacecraft鈥檚 heat-shield.
鈥淚t鈥檚 a drastic assumption, but we have to make it,鈥� said Ron Dittemore, manager of the space shuttle program, at the Johnson Space Center, Houston, on Monday.
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Many thousands of tiles line the underside of the shuttle to protect it against the intense heat of reentry into the Earth鈥檚 atmosphere. Dozens of the tiles are routinely damaged or lost during every shuttle flight, apparently without causing problems.
The external fuel tank holds the liquid hydrogen and oxygen fuel required for launch. It is held at very low temperatures and polyurethane foam insulation is needed to prevent the build up of ice, which, being heavier, would have an even greater capacity to damage the shuttle.
Three hundred hits
In 1992, Columbia was struck in an almost identical incident to that revealed in images from its ill-fated final launch on 16 January. A piece of foam insulation fell from the external fuel tank and hit the underside of the shuttle, gouging a 12-centimetre-long hole in the heat tiles.
This incident was not thought serious. 鈥淚t never represented any more than a threat to delay turnaround operations,鈥� said Dittemore.
Columbia was damaged again during a 1997 mission. A NASA report from the time said: 鈥淚nspection revealed more than 300 hits to Columbia鈥檚 exterior thermal protection system (TPS) tiles, with about 132 measuring greater than one inch long. Current estimations indicate about 100 TPS tiles may need replacement.鈥� Again, NASA concluded: 鈥淭he damaged tiles posed no threat to flight crew safety.鈥�
Popcorning foam
In 1995, NASA had estimated that 90 per cent of the damage to tiles was being caused by pieces of the external fuel tank insulation flaking away. NASA believed that air bubbles trapped in the foam were heating and expanding during launch, causing the foam to fail and fall off.
NASA called the problem 鈥減opcorning鈥� and in 1999 modified the make up of the foam. This was believed to have solved the problem.
But during an October 2002 launch of shuttle Atlantis, a piece of insulation once again fell from the external fuel tank and struck a solid rocket booster. The damage was superficial. 鈥淲e decided it did not represent a safety risk to us,鈥� said Dittemore.
If the impact of the insulation is found to be the root cause of the Columbia disaster, then the task facing NASA will be to understand why something that had happened many times before without any problems became a fatal flaw this time round.